Re: New filesystem for Linux

[Eric Dumazet <dada1@cosmosbay.com>]

Gautham R Shenoy a écrit :
> On Thu, Nov 02, 2006 at 10:52:47PM +0100, Mikulas Patocka wrote:
> > Hi
>
> Hi Mikulas
> > As my PhD thesis, I am designing and writing a filesystem, and it's now in 
> > a state that it can be released. You can download it from 
> > http://artax.karlin.mff.cuni.cz/~mikulas/spadfs/
> >
> > It has some new features, such as keeping inode information directly in 
> > directory (until you create hardlink) so that ls -la doesn't seek much, 
> > new method to keep data consistent in case of crashes (instead of 
> > journaling), free space is organized in lists of free runs and converted 
> > to bitmap only in case of extreme fragmentation.
> >
> > It is not very widely tested, so if you want, test it.
> >
> > I have these questions:
> >
> > * There is a rw semaphore that is locked for read for nearly all 
> > operations and locked for write only rarely. However locking for read 
> > causes cache line pingpong on SMP systems. Do you have an idea how to make 
> > it better?
> >
> > It could be improved by making a semaphore for each CPU and locking for 
> > read only the CPU's semaphore and for write all semaphores. Or is there a 
> > better method?
>
> I am currently experimenting with a light-weight reader writer semaphore 
> with an objective to do away what you call a reader side cache line
> "ping pong". It achieves this by using a per-cpu refcount.
>
> A drawback of this approach, as Eric Dumazet mentioned elsewhere in this
> thread, would be that each instance of the rw_semaphore would require
> (NR_CPUS * size_of(int)) bytes worth of memory in order to keep track of
> the per-cpu refcount, which can prove to be pretty costly if this
> rw_semaphore is for something like inode->i_alloc_sem.

We might use an hybrid approach : Use a percpu counter if NR_CPUS <= 8

#define refcount_addr(zone, cpu) zone[cpu]

For larger setups, have a fixed limit of 8 counters, and use a modulo

#define refcount_addr(zone, cpu) zone[cpu & 7]

In order not use too much memory, we could use kind of vmalloc() space, using 
one PAGE per cpu, so that addr(cpu) = base + (cpu)*PAGE_SIZE;
(vmalloc space allows a NUMA allocation if possible)

So instead of storing in an object a table of 8 pointers, we store only the 
address for cpu0.


> So the question I am interested in is, how many *live* instances of this
> rw_semaphore are you expecting to have at any given time?
> If this number is a constant (and/or not very big!), the light-weight
> reader writer semaphore might be useful.
>
> Regards
> Gautham.

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